1. Field of the Invention
This invention relates generally to information signal recording and reproducing apparatus and, more particularly, is directed to a noise reduction circuit for reducing noise generally accompanying a reproduced information signal in an information signal recording and reproducing apparatus.
2. Description of the Prior Art
Noise reduction circuits for reducing noise and distortion which accompany a reproduced information signal are well known in the art. Such noise reduction circuits are designed to increase the dynamic range of the signal that can be recorded and reproduced from a recording medium such as a magnetic tape. In a typical noise reduction circuit, an encoder is provided for those signals which are to be recorded, and a complementary decoder is provided for those signals which are reproduced. The encoder generally includes a level compression circuit and a high frequency pre-emphasis circuit, wherein high frequency components of an information signal to be recorded are emphasized and the information signal is compressed with the compression level being inversely related to the information signal level. The decoder generally includes a level expansion circuit and a high frequency de-emphasis (low frequency emphasis) circuit to perform a complementary operation on the information signals which are reproduced.
A problem that results with conventional noise reduction systems is that they may be subject to so-called noise modulation. With noise modulation, noise components are varied as a function of input signal level variations. Such change in the noise components, or noise modulation, is highly perceptible and is quite distracting when it accompanies a reproduced audio signal. This phenomenon is produced when the frequency components of the input signal are noticeably different from the noise frequency components. For example, if the information signal is an audio signal representing the sound of a piano, noise modulation is heard separately and distinctly, and is not masked even if the volume level of the information signal is increased.
One proposal for reducing noise modulation in a noise reduction circuit is described in U.S. Pat. No. 4,162,462. In this proposal, the high frequency components of the information signal are pre-emphasized prior to recording when the information signal exhibits low and intermediate signal levels, and relatively little pre-emphasis is provided when the information signal exhibits higher levels. When the information signal processed in the foregoing manner is reproduced, the high frequency components are subjected to relatively high de-emphasis when the reproduced signal exhibits low and intermediate signal levels, and these high frequency components are subjected to relatively low de-emphasis when the reproduced signal is at a higher level. Although this proposal reduces the undesired effects of noise modulation, saturation of the magnetic record medium due to overshoot in the compressed signal nevertheless is present.
Accordingly, a noise reduction circuit which reduces noise modulation and also prevents transient saturation of the record medium has been proposed in U.S. patent application Ser. Nos. 06/226,821 and 06/226,822, both filed on Jan. 21, 1981 and both having a common assignee herewith. Thus, for example, in the encoder circuit of the first-mentioned U.S. patent application Ser. No. 06/226,821, a low level audio input signal has its high-frequency components pre-emphasized, but high level signals are processed with a substantially flat frequency characteristic. In other words, the input-output level characteristic of the encoder varies with frequency when the input signal level is low. However, due to differences in sensitivity of different magnetic tapes and/or different recording and reproducing apparatus with which a magnetic tape may be used, in noise reduction circuits of the above type in which the input-output level characteristic thereof varies with frequency, a level deviation may result between the level of the encoded signal which is recorded on a magnetic tape by means of a tape recording apparatus and the level of the reproduced output signal from the tape. This variation may result in unsatisfactory reproduction of the audio signal when processed by a corresponding decoder circuit. Practically speaking, this means that the dynamic range within which the audio signal can be processed must be reduced.